A is number is divisible by 11? Think of the numbers you know. Think of what happens when you multiply a number by 11. Or google for it. But the rule is like this: add every other digit, add the ones you skip. Substract the two numbers you got. If you get a multiple of 11, then the original number was divisible by 11

Example

123456789

Add the odd numbers. You get 25Add the even numbers. You get 20Substract them. The difference is not a multiple of 11. Hencem the number is not a multiple of 11